The number of antennas utilized in modern wireless devices (e.g. smartphones) are increasing in order to support new cellular bands, with bands now ranging between 600 MHz to 3800 MHz, multiple-input multiple-output (MIMO), diversity, carrier aggregation, wireless local area networks (WLANs), near field communication (NFC), global navigation satellite systems (GNSS), or other radio communication technologies, for example, which poses a challenge due to the volume or space required for each antenna to achieve good performance. For example, the performance of antennas in mobile devices is (among others) related to the volume or space allocated and the physical placement in the mobile device, such as a mobile phone. Increasing the allocated volume for the antenna can result in better antenna performance, for example, in terms of the reflection coefficient and/or the radiated efficiency. The width of the display is often nearly as wide as the smartphone itself, batteries take up a considerable volume inside the mobile device housing, and the available volume for antennas especially close to the outer casing of the housing is very limited and in many cases not usable for antennas also as a result of coupled interference. Other components like the USB connector, the audio jack and different user control buttons, are normally also placed at the outer casing of the housing, reducing the available volume for the antenna within the housing even more. Therefore, it is desired to provide antenna modules with low volume consumption and good performance for wireless communication devices.
In order to achieve high data rate and improve the radio link performance a wide spread adoption of MIMO technology is being utilized in the long term evolution (LTE) standards. MIMO antennas utilize the richness of the multipath wireless channel to enable multiple parallel data streams; nonetheless, they can rely on independent channel realizations. However, in a mobile phone bellow 1 GHz, the antenna elements are electrically small and rely on the ground plane to radiate efficiently. The sharing of the ground plane results in naturally strongly coupled elements, which can lead to reduced total efficiency (TE) and very high envelope correlation coefficient (ECC) due to similar radiation patterns, and thus poor MIMO performance. According to industrial research reports, an ECC less than 0.5 and a total efficiency higher than 40% can be rules of thumb for designing cellular LTE MIMO antennas in the lower bands. Thus, it is crucial to mitigate the mutual coupling and develop an adequate isolation mechanism that can be reliable, versatile and practical.